1. Field of the Invention
This invention generally relates to digital communications and, more particularly, to a system and method for minimizing the effects of intersymbol interference in a data channel of binary coded information.
2. Description of the Related Art
FIG. 1 is a diagram illustrating a signal recovered from a binary symmetric, non-dispersive channel in the presence of noise (prior art). Conventionally, the signal is filtered with a transfer function matched to the signaling waveform (in this case a one unit step) and thresholded at the voltage level most likely to yield the transmitted bit. To recover the transmitted information, a hard decision must be made on the value of the received bit. As the rates of high speed serial channels become higher and higher, intersymbol interference (ISI) becomes the limiting factor for reliable data transmission.
As a function of the filtering process, and sometimes as a result of the transmission process, pulse spreading (ISI) occurs. That is, the energy associated with a bit spreads to neighboring bits. For small degrees of spreading these effects of this can be limited to the nearest neighbors with modest degradation in performance. The ISI may be caused by various non-ideal effects of the transmission channels, such as bandwidth frequency selective behavior in backplane lines (e.g., FR4), or optical dispersion in the optical fiber.
After conversion from analog to digital signals, the ISI can practically seen as associated with one of three possible pulse spreading errors. The first possibility is that both the neighboring bits are a zero (no neighboring bits are a one). The second possibility is that only one of the neighboring bits (either the preceding or subsequent bit) is a one. Alternately stated, only one of the neighboring bits is a zero. The third possibility is that both neighboring bits are one. For each of these cases the likelihood of error in determining a bit value can be minimized if the high frequency components of the signal can be accurately recovered.
FIG. 2 is a diagram illustrating received waveforms that are distorted in response to the intersymbol interference resulting from energy dispersion (prior art). The value at the output of the filter varies with each bit, and is essentially a random process, due to the non-deterministic nature of the information, and scrambling that is often used in the transmission of data streams. However, received bits can be characterized with probability density functions (PDFs), as shown.
Using a conditional feed-forward equalizer (FFE) with a decision feedback equalizer (DFE), bit value decisions can be made using the knowledge of the decision made on the preceding decoded bit and with a measurement of a subsequent decoded bit. Then, the corresponding probability density function can be selected to make a more accurate decision on the current bit decision. However, the cost and accuracy of conventional analog-to-digital (A/D) conversion circuits make such a solution impractical.
The degree of dispersion exhibited by a channel, and hence the separation of the conditional probability density functions, varies in response to a number of fixed and variable factors. Effective dispersion mitigation techniques must therefore be easily optimized to the channel and somewhat adaptive to changes in the channel due to aging, temperature changes, reconfiguration, and other possible influences.
Equalization is an effective method to combat the ISI and it has been used for various communication channels. The implementation of equalizers can be performed in either the digital or analog domains. Finding an efficient implementation of equalization can be especially challenging at rates greater than 10 gigabits per second (GB/s). Further, high speed CMOS circuits typically require more power, as the current for the enabling CMOS circuitry increases non-linearly at higher operating frequencies.
It would be advantageous if intersymbol interference caused by energy dispersion in a received data channel could be minimized.
It would be advantageous if an equalization process could be developed that was both simple and energy efficient.